In the oil and gas exploration and production industry, a wellbore or borehole of an oil or gas well is typically drilled from surface to a first depth and lined with a steel casing which is cemented in place. The borehole is then extended and a further section of tubing known as a liner is located in the borehole, extending from the casing to a producing formation, and is also cemented in place. The well is then completed by locating a string of production tubing within the casing/liner, through which well fluids flow to surface.
However, before the well can be completed, it is necessary to clean the lined wellbore and replace the fluids present in the wellbore with a completion fluid such as brine. The cleaning process serves to remove solids adhered to the wall of the casing or liner; to circulate residual drilling mud and other fluids out of the wellbore; and to filter out solids present in the wellbore fluid. A considerable amount of debris in the wellbore and on the surface of the casing/liner comprises rust particles and metal chips or scrapings originating from equipment used in the well and the casing or liner itself.
Various types of cleaning tools are known, one of which is generically referred to as a casing scraper. Tools of this type typically incorporate casing scraper blades designed to scrape the inner surface of the casing/liner, for removing relatively large particles or debris from the surface of the tubing. Whilst it is recognised that it is desirable to utilise such cleaning tools to clean the casing/liner, when a casing scraper is removed from the well, the scraper blades can dislodge further debris into the wellbore fluid, negating the effect of cleaning procedures previously carried out. Similar difficulties have been encountered with other types of cleaning tools, including those having brushes or other abrading surfaces, circulation tools and the like.
In an effort to overcome disadvantages associated with the use of such tools, magnetic well cleaning apparatus has been developed, such as that disclosed in the Applicant's UK Patent Number 2,350,632, which includes a number of magnets. Another magnetic fishing tool is described in U.S. Pat. No. 6,591,117, wherein, large bar magnets are spaced apart around and along a tool body for the purposes of attracting and retrieving metal debris. These magnets may be permanent magnets made of any suitable magnetic material, such as neodymium iron boron, ceramic ferrite, samarium cobalt, or aluminium nickel cobalt. The bar magnets are fitted into recesses in the tool body and arranged to have an area between each magnet for metallic debris to settle. A further such tool is described in U.S. Pat. No. 6,354,386, wherein arcuate magnet assemblies are detachably secured by screws or other similar means to a body to be mounted in a drill string. An alternative fastening arrangement described there for the magnet assemblies uses split retainer rings provided with locking members for securing the magnets on the body.
In use of such fishing tools, ferrous metal and debris present in the wellbore is attracted to the magnets and carried out of the wellbore when the cleaning tool is removed or “tripped” from the well.
An object of the invention is to provide further improvements in tool assembly and design, and in particular one of the objects of the present invention is to provide an improved wellbore cleaning tool. A further object of the invention is to provide improvements in devices for retention of magnets on a wellbore cleaning tool.